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An ultimate one-dimensional electronic channel in hexagonal boron nitride

Posted By Graphene Council, Wednesday, March 11, 2020
In the field of 2D electronics, the norm used to be that graphene is the main protagonist and hexagonal boron nitride (hBN) is its insulating passive support. Researchers of the Center for Multidimensional Carbon Materials (CMCM) within the Institute for Basic Science (IBS, South Korea) made a discovery that might change the role of hBN. They have reported that stacking of ultrathin sheets of hBN in a particular way creates a conducting boundary with zero bandgap. In other words, the same material could block the flow of electrons, as a good insulator, and also conduct electricity in a specific location. Published in the journal Science Advances, this result is expected to raise interest in hBN by giving it a more active part in 2D electronics.

Similarly to graphene, hBN is a 2D material with high chemical, mechanical and thermal stability. hBN sheets resemble a chicken wire, and are made of hexagonal rings of alternating boron and nitrogen atoms, strongly bound together. However, unlike graphene, hBN is an insulator with a large bandgap of more than five electronVolts, which limits its applications.

“In contrast to the wide spectrum of proposed applications for graphene, hexagonal boron nitride is often regarded as an inert material, largely confined as substrate or electron barrier for 2D material-based devices. When we began this research, we were convinced that reducing the bandgap of hBN could give to this material the versatility of graphene,” says the first author, PARK Hyo Ju.

Several attempts to lower the bandgap of hBN have been mostly ineffective because of its strong covalent boron-nitrogen bonds and chemical inertness. IBS researchers in collaboration with colleagues of Ulsan National Institute of Science and Technology (UNIST), Sejong University, Korea, and Nanyang Technological University, Singapore, managed to produce a particular stacking boundary of a few hBN layers having a bandgap of zero electronVolts.

Depending on how the hBN sheets are piled up, the material can assume different configurations. For example, in the so-called AA′ arrangement, the atoms in one layer are aligned directly on the top of atoms in another layer, but successive layers are rotated such that boron is located on nitrogen and nitrogen on boron atoms. In another type of layout, known as AB, half of the atoms of one layer lie directly over the center of the hexagonal rings of the lower sheet, and the other atoms overlap with the atoms underneath.

For the first time, the team has reported atomically sharp AA′/AB stacking boundaries formed in few-layer hBN grown by chemical vapor deposition. Characterized by a line of oblong hexagonal rings, this specific boundary has zero bandgap. To confirm this result, the research performed several simulations and tests via transmission electron microscopy, density functional theory calculations, and ab initio molecular dynamics simulations.

“An atomic conducting channel expands the application range of boron nitride infinitely, and opens new possibilities for all-hBN or all 2D nanoelectronic devices,” points out the corresponding author LEE Zonghoon.

Tags:  2D materials  Graphene  hexagonal boron nitride  Institute for Basic Science  PARK Hyo Ju 

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Study puts spin into quantum technologies

Posted By Graphene Council, Thursday, February 27, 2020

A team of international scientists investigating how to control the spin of atom-like impurities in 2D materials have observed the dependence of the atom's energy on an external magnetic field for the first time.

The results of the study, published in Nature Materials, will be of interest to both academic and industry research groups working on the development of future quantum applications, the researchers say.


Researchers led by Prof Vladimir Dyakonov at the University of Würzburg in collaboration with scientists from the University of Technology Sydney (UTS), the Kazan Federal University and the Universidade Federal de Minas Gerais, demonstrated the ability to control the spin of atom-like impurities in 2D material hexagonal boron-nitride. By combining laser and microwave excitation the researchers were able to change the spin states, for example "up" to "down", of atom-like impurities hosted in the material and show the dependence of their energy on an external magnetic field.

This is the first time that the phenomenon has been observed in a material that is made of a single sheet of atoms like graphene. The researchers say that this newly demonstrated quantum spin-optical properties, combined with the ease of integrating with other 2D materials and devices, establishes hexagonal boron-nitride as an intriguing candidate for advanced quantum technology hardware.

"2D atomic crystals are currently some of the most studied materials in condensed matter physics and materials science," says UTS physicist Dr Mehran Kianinia, a co-author of the study.

"Their physics is intriguing from a fundamental point of view, but beyond that, we can think of stacking different 2D crystals to create completely new materials, heterostructures and devices with specific designer properties," he says.

UTS researcher, Dr Carlo Bradac, a senior co-author of the study says that in addition to adding another unique property, to an already impressive range of properties for a 2D material, the discovery has enormous potential for the field of quantum sensing.

"What really excites me is the potential [in the context of quantum sensing]. These spins are sensitive to their immediate surroundings. Unlike 3D solids, where the atom-like system can be as far as a few nanometres from the object to sense, here the controllable spin is right at the surface. Our hope is to use these individual spins as tiny sensors and map, with unprecedented spatial resolution, variations in temperature, as well as magnetic and electric fields onto variations in spin" Dr Bradac says.

"Imagine, for instance, being able to measure minuscule magnetic fields with sensors as small as single atoms. The possibilities are far reaching and range from nuclear magnetic resonance spectroscopy for nanoscale medical diagnostic and material chemistry to GPS-free navigation using the Earth's magnetic field," he says.

However quantum-based nanoscale magnetometry is "just one area where controlling single spins in solids is useful" says senior author of the study UTS Professor Igor Aharonovich.

"Beyond quantum sensing, many quantum computing and quantum communication applications rely on our ability to control the spin-state--zero, one and anything in between--of single atom-like systems in solid host materials. This allows us to encode, store and transfer information in the form of quantum bits or qubits," he says.

Amongst many others, this research highlights how scientists are quickly becoming masters in the craft of manipulating objects in the quantum regime. In fact, achievements like Lockheed Martin's Black Ice project and Google's quantum supremacy are proof that we are striding away from mere proof-of-concept experiments towards real world, quantum-enabled solutions to practical problems.

Tags:  2D materials  Graphene  Hexagonal boron nitride  Kazan Federal University  Nature Materials  Universidade Federal de Minas Gerais  University of Technology Sydney  University of Wurzburg  Vladimir Dyakonov 

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How tiny misalignments in encapsulated graphene lead to a strong modification of its electronic properties

Posted By Graphene Council, Friday, January 31, 2020
Researchers at the University of Antwerp explain how higher order supermoiré periodic modulations due to the encapsulation of graphene between hexagonal boron nitride affect the electronic and structural properties of graphene, as revealed in three recent independent experiments.

High quality graphene samples are of high importance for obtaining and exploiting its theoretically described properties. Utilizing an adequate substrate reduces the corrugation and improves otherwise disorder limited properties of graphene.

Hexagonal boron nitride (hBN) is a particularly good choice, since it preserves perfectly the graphene structure, while providing a flat insulating surface. Still, this applies only if the two monolayers are misaligned. Otherwise, the van der Waals interaction induces structural relaxation on the scale of the moiré pattern formed between the two layers and modifies the electronic properties due to the periodic moiré perturbation.

Similar arguments apply if graphene is encapsulated and closely aligned to two hBN layers. In this case the effect is enhanced since both layers are expected to contribute. Furthermore, close alignment, on the order of 0.5 degrees, between the layers is responsible for the appearance of a new form of periodic supermoiré modulation, which alters graphene on a larger spatial scale, but smaller energy scale.

Recent experimental observation of such effects are a consequence of significant improvements in the experimental manipulation techniques, and among others, the possibility to rotate individual layers with high precision (Wang et al. 2019a; Wang et al. 2019b; Finney et al. 2019 – see references at the end of this article).

In their recent paper published in Nano Letters ("Double moiré with a twist: supermoiré in encapsulated graphene"), Anđelković et al. reveal under which condition the supermoiré effect appears, and how it alters the structural and electronic properties of graphene.

They show, starting from a rigid hBN/graphene/hBN heterostructure, how the supermoiré appears as a simple geometrical consideration. Furthermore, they prove that relaxation effects in the three layers are expected to enhance the effects on the electronic band structure. The supermoiré induced modifications are significant: new low energy flat sub-bands and Dirac points appear, with strong effect on electronic transport properties. In most configurations the Dirac points are gapped, while flat bands are expected to enhance electron-electron correlations.

"These new twisting degrees of freedom in heterostructures are opening up new fundamental research directions in graphene, where strong electronic correlations are expected to complement the already superlative properties of graphene," said Dr. Lucian Covaci.

"The set of multi-scale numerical simulations developed by the University of Antwerp team allows for more realistic models, which will in turn allow for a more direct comparison with experimental observations," said Dr. Miša Anđelković, a co-developer of Pybinding, the tight-binding open source software that made the simulations possible.

With a new light shed on the understanding of more complex and interfering behaviour of van der Waals heterostructures it is possible to finely tune graphene’s electronic properties and reach regimes where twist induced phenomena, such as flat bands or the appearance of mini-gaps, reveal themselves more clearly.
 

Tags:  Graphene  hexagonal boron nitride  Lucian Covaci  Miša Anđelković  University of Antwerp 

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Researcher’s break the geometric limitations of moiré pattern in graphene heterostructures

Posted By Graphene Council, Wednesday, January 1, 2020
Researchers at The University of Manchester have uncovered interesting phenomena when multiple two-dimensional materials are combined into van der Waals heterostructures (layered ‘sandwiches’ of different materials).

These heterostructures are sometimes compared to Lego bricks – where the individual blocks represent different atomically thin crystals, such as graphene, and stacked on top of each other to form new devices.

Published in Science Advances, the team focus on how the different crystals begin to alter one another’s fundamental properties when brought into such close proximity. Of particular interest is when two crystals closely match and a moiré pattern forms. This moiré pattern has been shown to affect a range of properties in an increasing list of 2D materials. However, typically the geometry of the moiré pattern places a restriction on the nature and size of the effect.

A moiré pattern is due to the mismatch and rotation between the layers of each materials which produces a geometric pattern similar to a kaleidoscope.

Our results push through the geometric limitation for these systems and therefore present new opportunities to see more of such science, as well as new avenues for applications.
Zihao Wang and Colin Woods, School of Natural Science

The team have broken this restriction by combining moiré patterns into composite ‘super-moiré’ in graphene both aligning to substrate and encapsulation hexagonal boron nitride. The researchers demonstrate the nature of these composite super-moiré lattices by showing band structure modifications in graphene in the low-energy regime. Furthermore, they suggest that the results could provide new directions for research and devices fabrication.

Zihao Wang and Colin Woods authors of the paper said: “In recent years moiré pattern have allowed the observation of many exciting physical phenomena, from new long-lived excitonic states, Hofstadter’s Butterfly, and superconductivity.

Our results push through the geometric limitation for these systems and therefore present new opportunities to see more of such science, as well as new avenues for applications.”

Tags:  2D materials  Colin Woods  Graphene  hexagonal boron nitride  University of Manchester  Zihao Wang 

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Saving Moore’s Law

Posted By Graphene Council, Tuesday, December 31, 2019
It’s a well-known observation: The number of transistors on a microchip will double roughly every two years. And, thanks to advances in miniaturization and performance, this axiom, known as Moore’s Law, has held true since 1965, when Intel co-founder Gordon Moore first made that statement based on emerging trends in chip manufacturing at Intel. 

However, integrated circuits are hitting hard physical limits that are rendering Moore’s Law obsolete — elements on a dense integrated circuit (IC) can get only so small and so tightly packed together before they begin to interfere with each other and otherwise lose their functionality.

“Apart from fundamental physical limits to the scaling of transistor feature sizes below a few nanometers, there are significant challenges in terms of reducing power dissipation, as well as justifying the incurred cost of IC fabrication,” said Kaustav Banerjee, a professor of electrical and computer engineering at UC Santa Barbara. As a result, the very devices that we rely on for their steadily improving performance and versatility — computers, smartphones, internet-enabled gadgets — would also hit a limit, he said.

But according to Banerjee, one of world’s leading scientific minds in the field of nanoelectronics, there is a way to maintain Moore’s Law indefinitely, by taking advantage of relatively new and promising two-dimensional (2D) materials and combining them with monolithic 3D (M3D) integration practices to create ultra-compact, yet high-performing electronic chips that could overcome the challenges that face conventional integrated circuits. While Banerjee first disclosed this idea in a visionary article back in 2014, more detailed research evaluating this technology from his Nanoelectronics Research Lab was recently published in the IEEE Journal of the Electron Devices Society.

“Two-dimensional materials can be stable in their monolayer form with atomic scale thickness – 0.5 nanometer or 5 Angstroms for graphene (a conductor) and hexagonal-boron-nitride (an insulator), and ~6.5 Angstroms for 2D transition metal dichalcogenides (semiconductors) such as molybdenum-disulphide (MoS2) or tungsten-disulphide/diselenide (WS2/WSe2).” Banerjee said. “In addition, due to their layered nature, they offer pristine surfaces relatively free of defects and are excellent conductors of heat in the in-plane direction. All these properties, along with the possibility to directly synthesize these materials on top of prefabricated devices, offer unprecedented advantages over conventional 3D ICs that are already in the market or M3D integration with conventional electronic materials.”

The Benefits of Thinness 

According to the Banerjee Group’s study, there’s a limit to how thin conventional semiconductor materials can get before their desirable electronic properties begin to fade. 

“Thickness scaling of common semiconductor materials, such as Si, becomes challenging below a few nanometers due to rapid degradation of their mobility caused by the increase in electron scatterings from surface roughness,” Banerjee said. “In fact, below ~1 nm, conventional materials like Si or Ge may not be thermodynamically stable.”

On the other hand, atomically thin and stable 2D materials, such as graphene, hexagonal boron nitride (h-BN), and transitional metal dichalcogenides (MoS2, WS2, WSe2, etc) are highly space-efficient, thickness-wise. Moreover, due to their layered nature and pristine interfaces, the 2D semiconductors exhibit reasonably high mobilities and immunity against surface defects, according to the paper. In addition, 2D materials tend to be a lot more flexible than their conventional counterparts, which make them ideal for state-of-the-art electronics applications, such as flexible displays.  Stacked 2D materials, in contrast to their stacked 3D counterparts, meanwhile, can also minimize the inter-tier signal delays, thermal resistance, and reduce potential overheating.

By selecting certain 2D materials and stacking them, according to the researchers, not only does the monolithic 3D conserve precious space on the chip, but also allows for configuration based on the combined electronic properties of the materials.

For example, owing to the atomically-thin vertical dimensions of 2D materials, and carefully-designed inter-tier electrostatics with graphene shielding layer that also benefits from enhanced heat dissipation, aggressive scaling of tier thickness down to sub-μm can be achieved,” Banerjee said. “Such scaling allows over 10-folds higher integration density with respect to conventional 3D integration, and over 150% greater integration density with respect to conventional M3D integration, with plenty of room for further improvements.” 

“Thus, 2D materials can help realize the ultimate density scaling of integrated electronics — both laterally and vertically — which can usher an unprecedented era of innovation and economic growth for the worldwide semiconductor industry,” he added.

Manufacturing Outlook

As with many innovations with potential to become mainstream technologies, there are challenges to consider to pave the way toward their mass manufacturing. For monolithic 3D devices, the challenges are to be able to fabricate these components at relatively low temperatures (lower than 500 degrees Celsius) to avoid degradations and damages to prefabricated devices located in the lower tiers; electromagnetic interference; and heat dissipation.

Last year, Banerjee’s group demonstrated a CMOS compatible graphene synthesis method that essentially addressed the low-temperature and transfer-free synthesis challenge for graphene. Similar efforts are underway in his laboratory to synthesize other 2D materials directly on wafers at low temperatures.

“Additionally, careful design is needed to electrically shield the generated electromagnetic waves from affecting the operations of devices on adjacent or nearby tiers,” said Junkai Jiang, the lead author of the article and recent recipient of a doctoral degree in electrical and computer engineering from Banerjee’s laboratory. The researchers noted that by using a thin graphene shielding layer between tiers (preferably doped to enhance electromagnetic screening effect), interference can be prevented even as the vertical layers are scaled down. 

In terms of heat dissipation, the thinness of the material itself is conducive to allowing the heat from densely packed stacked components to dissipate efficiently. Kamyar Parto, a co-author of the study and a member of Banerjee’s lab, remarked that “the 2D materials have much higher in-plane thermal conductivity compared to thinned-down conventional materials like silicon, which helps fast lateral heat transport, thereby reducing the risks of any hot-spot formation.”  

“Ultimately, we envision heterogeneously integrated devices and technologies enabled by 2D materials to realize the world’s tallest and densest ‘chip-cities’ with unprecedented performance, storage capacity, and energy-efficiency,” he added.

Tags:  2D materials  Electronics  Graphene  Hexagonal boron nitride  Intel  Junkai Jiang  Kamyar Parto  Kaustav Banerjee  nanoelectronics  Semiconductor 

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Modified 'white graphene' for eco-friendly energy

Posted By Graphene Council, Wednesday, April 24, 2019
Updated: Tuesday, April 23, 2019
Scientists from TPU, Germany, and the United States have found a new way to functionalize a dielectric, otherwise known as 'white graphene', i.e. hexagonal boron nitride (hBN), without destroying it or changing its properties. Thanks to the new method, the researchers synthesized a 'polymer nano carpet' with strong covalent bond on the samples.

Prof Raul Rodriguez from the TPU Research School of Chemistry & Applied Biomedical Sciences explains:

'For the first time, we have managed to covalently functionalize hexagonal boron nitride without strong chemical compositions and the introduction of new defects into the material. In fact, earlier approaches had resulted in a different material with altered properties, i.e. hydrolyzed boron nitride. In our turn, we used nanodefects existing in the material without increasing their number, and eco-friendly photopolymerization.'

One of the promising options for using the new material, according to researchers, is catalysts for splitting water in hydrogen and oxygen. With this in view, 'polymer carpets' functioned as carriers of active substances, i.e. matrices. Nickel nanoparticles were integrated into the matrix. Catalysts obtained were used for electrocatalysis. Studies showed that they could be successfully used as an alternative to expensive platinum or gold.

'One of the important challenges in catalysis is forcing the starting material to reach active centers of the catalyst. 'Polymer carpets' form a 3D structure that helps to increase the area of contact of the active centers of the catalyst with water and makes hydrogen acquisition more efficient. It is very promising for the production of environmentally friendly hydrogen fuel,' - says the scientist.

Boron nitride is a binary compound of boron and nitrogen. While, hexagonal boron nitride or 'white graphene' is a white talc-like powder with hexagonal, graphene-like lattice. It is resistant to high temperatures and chemical substances, nontoxic, has a very low coefficient of friction, and functions both as a perfect dielectric and as a good heat conductor. Boron-nitride materials are widely used in the reactions of industrial organic synthesis, in the cracking of oil, for the manufacturing of products of high-temperature technology, the production of semiconductors, means for extinguishing fires, and so on.

Previously, a number of studies were devoted to functionalization of hexagonal boron nitride. Typically, this process uses strong chemical oxidants that not only destroy the material but also significantly change its properties. The method, which TPU scientists and their foreign colleagues use, allows them to avoid this.

'Studies have shown that we obtained homogenous and durable 'polymer carpets' which can be removed from the supporting substrate and used separately. What is more, this is a fairly universal technology since for functionalization we used different monomers which allow obtaining materials with properties optimal for use in various devices,' - says Prof Raul Rodriguez.

Tags:  2D materials  Graphene  Hexagonal Boron Nitride  Raul Rodriguez  TPU Germany 

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Graphene Changes the Game in Optoelectronics

Posted By Dexter Johnson, IEEE Spectrum, Tuesday, April 24, 2018

Photons are faster than electrons. This has lead scientists to see if they can harness light (photons) to operate an integrated circuit. While this should result in faster circuits, there’s a hitch: wavelengths of light are much larger than the dimensions of today’s computer chips. The problem is that you simply can’t compress the wavelengths to the point where they work in these smaller chip-scale dimensions.

Scientists have been leveraging a new tool lately to shrink the wavelengths of light to fit into smaller dimensions: plasmonics. Plasmonics exploits the waves of electrons—known as plasmons—that are formed when photons strike a metallic structure. Graphene has played a large role in this emerging field because it has the properties of a metal—it’s a pure conductor of electrons.

The Institute of Photonic Sciences (ICFO) in Barcelona,  which has been a leader in this field for years, is now reporting they have taken the use of graphene for shrinking the wavelengths of light to a new level. In research described in the journal Science, ICFO researchers have managed to confine light down to a space one atom thick in dimension. This is certainly the smallest confinement ever achieved and may represent the ultimate level for confining light.

The way the researchers achieved this ultimate confinement was to use graphene along with one of its two-dimensional (2D) cousins: hexagonal boron nitride, which is an  insulator.

By using these 2D cousins together, the researchers created what’s known as van der Waals heterostructures in which monolayers of different 2D materials are by stacked on top of each other and held together by van der Waal forces to create materials with tailored electronic properties—like different band gaps for stopping and starting the flow of electrons. In this case, the layers included hexagonal boron nitride layered on top of the graphene and then involved adding an array of metallic rods on top of that. This structure had the graphene sandwiched between an insulator and a conductor. The graphene in this role served to guide the plasmons that formed when light struck the outer metallic rods.

In the experiment, the ICFO researchers sent infrared light through devices made from the van der Waal heterostructures to see how the plasmons propagated in between the outer metallic rods and the graphene.

To get down to the dimensions of one atom for confining the light, the researchers knew that they had to reduce the gap between the metal and the graphene. But the trick was to see if it was possible to reduce that gap without it leading to additional energy losses.

To their surprise, the ICFO researchers observed that even when a monolayer of hexagonal boron nitride was used as a spacer, the plasmons were still excited by the light, and could propagate freely while being confined to a channel of just on atom thick. They managed to switch this plasmon propagation on and off, simply by applying an electrical voltage, demonstrating the control of light guided in channels smaller than one nanometer of height.

The researchers believe that these results could to lead a new generation of optoelectronic devices that are just one nanometer thick. Down the road, this could lead to new devices such as ultra-small optical switches, detectors and sensors.

Tags:  graphene  Hexagonal boron nitride  optical switches  optoelectronics  plasmonics  sensors 

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GrollTex Tackles Sensor Markets With High Quality Graphene

Posted By Dexter Johnson, IEEE Spectrum, Thursday, March 1, 2018

  Jeffrey Draa, CEO, GROLLTEX

 

Last month, The Graphene Council's Executive Director, Terrance Barkan, and its Editor-in-Chief, Dexter Johnson, had the opportunity to have a talk with the CEO of California-based Grolltex Inc., Jeffrey Draa, about the company's business strategies in bringing graphene products to market and his views on graphene's future. Here is that conversation.

Could you tell us a little bit about the background of GrollTex. How did the company get started and how did you get involved with graphene? In particular, could you provide the history of Grolltex as a company?

Sure, so the name Grolltex is short for graphene rolling technologies and the brief history of the company is that my partner and co-founder and really the inventor, Dr. Alexander Zaretski, was a researcher at University of California San Diego.

He was involved with graphene growth and really got deep into graphene manufacturing techniques while he was at the University of California San Diego. One of the issues with this specific kind of graphene, as generated by chemical vapor deposition (CVD), of course, is the ‘transfer’ issue: How does one get single-layer graphene synthesized from copper off of the copper growth substrate and onto a substrate of interest without destroying the copper growth substrate? Of course, the current state-of-the art is to either acid etch the copper off of CVD graphene, or to use an electrolytic solution to sort of bubble the graphene off of the copper and have it rise to the top after a long period of time.

So both of these two processes, which had been state-of-the art, impact the copper in a very negative way so it's very expensive and not manufacturable. And my partner, Alexander, decided if graphene is going to go forward, there has to be a way to manufacture graphene and not destroy or impact that copper.  So he came up with a process to do that, a process that has a rolling schema where we reuse that growth copper over and over again. So that's kind of the background of the company. Alex had decided that he wanted, and felt so passionately about, this transfer technology and bringing it to graphene manufacturing that after completing his work as a researcher at UCSD, he broke out on his own and he asked me if I would be the business side of the company and he had the technical side. So that's kind of a brief background of Grolltex and how we came to be.

I understand you’re privately held company, correct?

We are, yes. We were funded roughly a year and a half ago with our seed funding and we've since about six months ago taken another round.

In terms of your graphene manufacturing that you just laid out, as you said you focused on producing single-layer graphene of the highest quality, so what are the markets that this product offering opens up to you? And what do you see as your strongest market now and do you see that market changing five years from now?

Well, as anybody that has knowledge of the graphene markets knows, single-layer high purity graphene like that synthesized via CVD has many theoretical use cases. We see on the short-term horizon three particular applications that are really kind of starting to command our attention. Those three are number one: sensing. So graphene given its electrical and mass properties makes an excellent sensor at a very, very small level. So sensing is number one.

We also are doing some work in the advanced solar cell arena and we have a grant from the California Energy Commission where we're working on a two-sided solar cell where graphene not only plays the part of barrier material but it's also the electrode material. So that's really exciting.

And for number three we’re also starting to get some inquiries for an application that actually Dr. Andre Geim at the University of Manchester, who, of course, was the discoverer of graphene was very passionate about. This is one of the very first applications that he thought futuristically would really make the world a better place, and that third application that we're starting to see on the horizon is graphene as a proton exchange membrane in a hydrogen fuel cell.

So those are kind of the three leading candidates we see right now. We’re judging that by some initial business that we’re getting in those areas.

You were discussing a number of applications you are pursing, including sensors. On your website you talk about enabling sensors that could be used for the Internet of Things. Can you explain why you see graphene playing such an important role in the development Internet of Things?

I’ll speak a little about graphene as a sensor material. When you combine the electrical conductivity properties along with the fact that graphene is one atom thick, you've got the potential for a sensor that could take us into the future for the next hundred years. We have patents around some designs of graphene-based sensing materials that are so sensitive that, for example, in the biotech world we had some bioengineering folks at Stanford use our sensor to sense the ability of individual heart cells to contract. Currently there only exists a different kind of test that can only count the number of contractions, but our sensor is so sensitive that it picks up the strength of contraction of the individual heart cell when it beats and it's a very robust signal; there's no mistaking it. So that's just one example of the potential of graphene as a sensor and we're seeing good activity there.

What is consistently your biggest challenge when you're talking to potential customers and convincing them how to use your product? Are they worried about pricing of graphene, the quality of product, a consistent supply chain? What stands out as one of the key issues that keeps coming up when you're speaking to these people?

So, I think the first consistent theme would surprise no one, and it's price. Almost any inquiry goes down along the lines of price, especially for a field like solar. If solar is implemented it’s going to need miles and miles of cheap graphene. Now the case of a sensor is not quite as price sensitive, but with regards to the big kind of large applications people think about like flexible displays and some of the other big idea changes for graphene those are really price sensitive. So price is the first one.

We don't get too many concerns with regards to the supply chain. Quality of product is sometimes discussed and that's partly because graphene is such a new field. But a lot of folks have what they are calling graphene and maybe debatably it is not. We don't necessarily have that problem because no one argues that single-layer graphene made by CVD is not graphene, so we don't have any discussion of the quality, but that sometimes can be an issue. So to kind of summarize, and get back to your main question, really price is the first thing that people want and that's the first hurdle you have to get over with almost everyone.

In addition to your graphene product, you're also producing hexagonal boron nitride (sometimes called white graphene). How do you see this material filling out your portfolio and what are the applications for this material that you're currently targeting and do you expect to develop other two-dimensional materials?

Hexagonal boron nitride is something we’re very excited about for several reasons. For the folks that aren't familiar with hexagonal boron nitride, you need to understand how it works with graphene. Graphene is, of course, the most conductive substance known at room temperature; it's on the order of seven times more conductive than copper depending on who you talk to. So as a conductor, graphene is really unparalleled. Now if you're going to design an electronic device of any type, of course, you worry about a conducting material because you can make the wire, the battery, and the switch with the conductive material. But the other thing you have to worry about is the insulating material. What are you going to use for the insulation for graphene? You have to separate the layers of the devices and hexagonal boron nitride is as good an electrical insulator as graphene is a conductor. And hexagonal boron nitride has a hexagonal pattern when it is synthesized in the proper way and that pattern lines up perfectly with the hexagonal lattice pattern of graphene so it also provides the strength benefit too. So it is really the ideal cousin of graphene. If you're an electronic designer, you're going to want both a conductor and an insulator and now we're going to be delivering both.

So that answers your first question and your second question, which was “are we going to develop other two dimensional materials?” As far as basic building blocks, we are going to rest on graphene and hexagonal boron nitride for a time because again those are your two basic building blocks: you need the insulation and the conducting but we also are developing other materials that go into specific devices. So, an example of this is the sensors I talked about that require some precious metal in small quantity—atomic quantity.  There are other materials involved when you go to make a specific device, but as far as the basic building blocks we're going to stand pat on graphene and hBN probably for a while.

What is your perspective on hybrid graphene materials? I am referring to this combination of a conductor and an insulator, or even a conductor with a semiconductor, and based on that will you look to develop those hybrids yourself or have your client make the next step in the value chain?

At the moment our clients are doing that work. Now I'm not going to say that we won't get into it, but we're going to be opportunistic with regard to that. With regard to opportunistic roads that we can go down today, our plate is pretty full, but there are several routes we can take. We are seeing folks in the semiconductor world, which is my background, starting to use other materials and creating devices out of some of those second and third level hybrids as you described and that's really exciting work. So we may get into some of that, but again we have a lot on our plate right now just based on what I described already.

I’d like to get your view of the overall industry over the short, middle and long term—five, ten, fifteen years expectations of graphene and the industry. And what is your strategy for best placing your company in the environment that you see developing?

With regard to our company, just saying the word “graphene” to a lot of people opens up so many thought patterns, channels and ideas that one of the things that's going to have to happen is the standards are going to need to be put in place fairly soon so that people can know what they're saying when they say “graphene”.

There's a lot of graphitic solutions out there and hybrids and powders and all kinds of things that debatably aren't graphene (of course, I would say that because of my company is involved in the area that there is no argument that it is pure graphene). But the point of that is we will need some standards and some nomenclature put in place to help take this whole field to the next level.

There's all kinds of great use cases for graphitic solutions that aren't graphene—great use cases, don't get me wrong—but let's make sure that we can assign proper nomenclature so people know what they are talking about and looking at. With regard to my company specifically, one of our challenges is picking our targets because again there are so many kinds of different opportunities. And when we first started out we decided we were going to have a two-phased approach to our business and we're doing the phase one part of that now.

Phase one for us is to make and sell graphene material as research material. So our core customer for our phase one is the university lab and commercial lab. So we sell graphene on copper substrates, on wafers, we sell graphene on customer specific substrates. You send us your substrate of interest and we’ll put our graphene on it and send it back to you. That phase one of our business that I just described to you is allowing us to pursue all kinds of exciting applications and some of them are helping us go in new directions. So, our challenge in the first five years I think is; number one stay on that phase-one path, get to profitability just as a business and number two really pick our paths carefully with regard to what are going to be the first real big market businesses out there in graphene—the ones that have paying customers.

So from a commercialization perspective, I think what you mention is that the majority, or is it basically all, of your customers are they in the testing or R&D category right now?

Yes, that’s fair to say. There's a population of big players in the industry that have their own graphene “skunkworks” that they're just not talking about. For example, I'm just going to throw some names around freely about big companies that we happen to know that do have graphene labs internally that it's just really very hush-hush. The reason we know this is because we know some of the people that have been hired out of other graphene places into these big companies. For example, Apple is one of them. They don't talk about it but they have a big graphene effort. Hewlett Packard is of one of those. Samsung is not bashful about their graphene efforts. So there are a lot of big companies where there is a lot of activity going on but nobody is talking about it so I think there's a lot more happening in graphene than people are even aware of because it’s not being leaked.

One of the things we’re very interested in doing as the Graphene Council is helping to act as a catalyst and accelerate commercialization.

One of the biggest obstacles to commercialization we’ve seen is simply the education of potential end users and consumers.  Can you talk a little bit about that? I mean as a company trying to educate potential clients one by one is a time consuming and expensive proposition.

What are the some of the other vertical markets or specific application areas—you mentioned sensors, of course? Are there some other specific areas where you think there's good commercial opportunity where we can help educate those populations?

The first one that comes to mind is the display market. So the display folks, of course, have been using indium tin oxide (ITO) as their core material for decades. ITO is really not a great material for them; it's expensive, it involves dirty mining and it is very prone to pollution when getting it out of the earth. It's also brittle which is why everyone’s display on their phone, their laptop, their television, all displays are brittle; they're like glass.

Graphene is actually a plug and play replacement for ITO and graphene enables flexible displays. So, the first big use case I can think of and that would be the most exciting and the most impactful for the most people is ITO replacement for making flexible displays.

But also I think it’s a use case that's pretty far down the road. It’s very price sensitive for one reason and number two there is a huge infrastructure with multiple large multinational corporations already in place and has been in place for decades with a big manufacturing schema, billions of dollars all lined up to process ITO, etc.. That's not going to shut off overnight and just accept a graphene replacement, right? So, from a price perspective and from an implementation perspective there are some challenges with ITO replacement, but I think that it strikes me as the kind of area where we can start hammering away at some of the existing thinking.

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The Graphene Council thanks Jeffrey Draa, CEO of GROLLTEX for his time and unique insights into the developing market for graphene. 

 

Tags:  chemical vapor deposition  Hexagonal boron nitride  ITO  photovoltaics  sensors 

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Plasmonics Without Light Just Flipped Nanophotonics on its Head

Posted By Dexter Johnson, IEEE Spectrum, Monday, October 23, 2017

The use of graphene in the growing field known as plasmonics—in which the waves of electrons known as surface plasmons that are generated when photons strike a metallic structure—has been transforming the world of photonics and optoelectronics, enabling the possibility of much smaller devices operated by photons rather than electrons.

The Graphene Council has covered the work being performed at one of the leading research institutes in the world in this field of plasmonics, the Institute of Photonic Sciences (ICFO) in Barcelona. 

We had the opportunity to visit ICFO last week and speak to a number of their researchers, which we will be sharing in the coming weeks. In particular, we spoke to F. Javier García de Abajo from the Nanophotonics Theory research group at ICFO,  who has proposed a revolutionary approach of exploiting graphene for plasmonics.

It’s worth providing a bit of background on the field of plasmonics before jumping to this latest research. The use of photons instead of electrons for something like an integrated circuit has the clear benefit that photons travel much faster than electrons, promising much faster devices. However, the use of light in these applications is limited by the relatively large size of wavelengths of light. Light is fast, but their wavelengths are much larger than nanometer-scale dimensions of most integrated circuits.

Plasmonics provides a way to convert that light—photons—into waves of electrons that can be tuned to have much smaller dimensions than those of light. The dimensions of these plasmon waves can be a hundred times smaller than the smallest wavelengths of light. This means that light can serve as the basis of photonic integrated circuits, but many more devices than that.

The field of plasmonics has really taken in off in the last half-decade, and ICFO has been at the forefront of a lot of that work, especially in using graphene to enable the effect. However, what Garcia de Abajo has proposed is a new theoretical approach to generate visible plasmons in graphene not from light but from tunneling electrons.

In research published in the journal ACS Photonics, Garcia de Abajo and his colleague Sandra de Vega have suggested that there are more efficient ways of generating surface plasmons on graphene than using an external light source and have instead shown through models that graphene plasmons can be efficiently excited via electron tunneling in a sandwich structure formed by two graphene monolayers separated by a few atomic layers of hexagonal boron nitride.

As mentioned, it’s possible to tune the size of the plasmon waves, especially graphene plasmons, which can be changed in size according to the amount of doping level (an addition of other materials). While high doping levels can push the wavelength of the graphene plasmons towards the visible range, these grpahene plasmons primarily reside in the mid-infrared region, which translates into a weak coupling between far-field light and graphene.

What de Vega and García de Abajo have proposed is a methodology for visible-plasmon generation in graphene that requires no light at all. Instead, plasmons are generated from tunneling electrons, which are electrons that are able to pass through a material on the quantum level that they could not otherwise pass through.

To achieve this photon-less plasmonics, the researchers propose a graphene–hexagonal boron nitride (hBN)–graphene sandwich structure. In their model, the hBN layer is 1-nm thick that is sandwiched between two graphene monolayers.

When the right amount of voltage (bias) is applied between the two graphene sheets, it produces tunneling electrons through the gap. The researchers discovered a particular voltage window in which the tunneling electrons lose energy through the excitation of a propagating optical plasmon rather than dissipate through coupling with the vibrations of the crystal lattice of hBN that carry heat, which are known as phonons, (low bias) or electron–electron interactions (high bias).

One of the side benefits of plasmonic devices that operate in this way—without the need for photons—can also be used in reverse as sensors. In this way when a change occurs in the graphene plasmon properties, that change could lead to a voltage readout.

Tags:  electrons  graphene  hexagonal boron nitride  ICFO  photonics  photons  plasmonics  sensors 

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2D materials - Graphene and hBN (hexagonal boron nitride) enhances methanol fuel cell performance

Posted By Terrance Barkan, Tuesday, November 29, 2016

This is an authorized reprint of a recent publication in Advanced Energy Materials journal (Impact Factor: 16) (http://dx.doi.org/10.1002/aenm.201601216), by Stuart M. Holmes (Reader) and Prabhuraj - (PhD student - http://www.prabhuraj.co.uk/) from the School of Chemical Engineering and Analytical Science, University of Manchester in collaboration with the School of Physics, reporting the usage of 2D materials in operating direct methanol fuel cells, showing zero resistance to protons enhancing cell performance, thereby opening the bottle neck for commercialization of fuel cells. 

The content published is the sole responsibility of the authors. 

Fuel cells are an interesting energy technology for the near future, as they aid in production of sustainable energy using hydrocarbons as fuels, such as methanol, ethanol, acetone etc by a simple oxidation-reduction reaction mechanism.

Among different liquid fuels, methanol is attractive as it has a higher energy density (compared to lithium ion batteries and hydrogen) and other features such as ease in handling, availability etc. Hence methanol fuel cells find their potential use in laptop chargers, military applications or other scenarios where the access to electricity is difficult.

However the wider spectrum of commercial potential for methanol systems is greatly hindered by methanol cross over occurring in the membrane area of fuel cells. This is defined as the passage of methanol from anode to the cathode through the membrane, hence creating short circuit and greatly affecting the fuel cell performance.

This is mitigated by using barrier layer, in addition to the membrane used. 

Figure 1: Schematic illustration of methanol fuel cell and structure of graphene

So far many materials have been used as a barrier layer in methanol fuel cells, where the proton conductivity is balanced with the methanol cross over. Proton conductivity is one of the dominant factors, where slight reduction in proton conductivity can influence the fuel cell performance to a large extent. All the materials reported in the literature to date have seen a reduction in proton conductivity though methanol cross over is reduced. 

It is known that Andre Geim and his co-workers (Nature, A.K. Geim et.al 2014), discovered proton transfer through single layer graphene and other 2D materials. Also graphene is known for its dense lattice packing structure, inhibiting the passage of methanol and other hydrocarbon based molecules across the membrane. However the actual application of these 2D materials in fuel cell systems has not yet been realized.

In this Advanced Energy Materials paper, the researchers have used single layer graphene and hBN, formed by chemical vapour deposition method, as a barrier layer in the membrane of methanol fuel cells. They have reported that this thinnest barrier layer ever used before shows negligible resistance to protons, at the same time reducing cross over, enhancing the cell performance by 50%. This is of significant interest, as this would lead to usage of 2D materials in fuel cells.

Based on the results of the research obtained, researchers have been granted EPSRC (Engineering and Physical Sciences Research council grant “Adventurers in Energy grant”) to pursue further research in this field. They have shown that as the surface coverage of the 2D material on the system improved, the performance improved.  This would lead to the usage of fuel cells, operating with high concentrated methanol fuels, as the current fuel cells suffer from cross over phenomena, with increased concentration. 

Moreover, this would pave the way for a membrane-less fuel cell system operating with higher efficiency. This technology could further be extended to other fuel cells types namely hydrogen fuel cells. Hydrogen fuel cells suffer from the usage of high cost humidifier, where the membrane needs to be humidified for improved proton conductivity. Whereas graphene, as reported in earlier studies, showed improved proton conductivity with temperature, without the need for humidifier systems. The future prospect could be realized in such a way that the fuel cells will make significant contribution to the future energy demand. 


Tags:  Fuel Cells  Graphene  hBN  Hexagonal boron nitride  Methane 

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